Photoplethysmography

ABSTRACT

A photoplethysmograph device includes a light source for illuminating a target object. A modulator drives the light source such that the output intensity varies as a function of a modulation signal at a modulation frequency. A detector receives light from the target object and generates an electrical output as a function of the intensity of received light. A demodulator with a local oscillator receives the detector output and produces a demodulated output, insensitive to any phase difference between the modulation signal and the oscillator, indicative of blood volume as a function of time and/or blood composition. A number of demodulators may be provided to derive signals from multiple light sources of different wavelengths, or from an array of detectors. The plethysmograph may operate in a transmission mode or a reflectance mode. When in a reflectance mode, the device may use the green part of the optical spectrum and may use polarizing filters.

The present invention relates photoplethysmography and in particular toa method and apparatus for measuring pulse rate, breathing rate andblood constituents in the human or animal body.

The word plethysmography is a combination of the Greek words Plethysmos,meaning increase, and graph, meaning write. A plethysmograph is aninstrument, method or apparatus used to measure the variations in bloodvolume in the body. Photoplethysmography (hereinafter also referred toas ‘PPG’) refers to the use of light to measure these changes in volume,and therefore a photoplethysmograph is an instrument, method orapparatus that uses light to perform these measurements.

Although the human or animal body is generally assumed to be opaque tolight, most soft tissue will transmit and reflect both visible andnear-infrared radiation. Therefore, if light is projected onto an areaof skin and the emergent light is detected after its interaction withthe skin, blood and other tissue, time varying changes of lightintensity having a relation with blood volume, known as theplethysmogram, can be observed. This time varying light intensity signalwill depend on a number of factors including the optical properties ofthe tissues and blood at the measurement site, and the wavelength of thelight source. The signal results because blood absorbs light and theamount of light absorbed, and hence the intensity of remaining lightdetected, varies in relation with the volume of blood illuminated.Variation in the plethysmogram is caused by the variation in bloodvolume flowing in the tissue.

This technique was introduced in 1937 by Hertzman. He was the first touse the term photoplethysmography and suggested that the resultantplethysmogram represented volumetric changes of blood in the skin'svessels.

The plethysmogram is usually described with respect to its AC and DCcomponents. The absorption of light by non-pulsatile blood, bone andtissue is assumed to be constant and gives rise to the DC component. TheDC component represents the volume of non-pulsatile blood below thesensor, plus light reflected and scattered off the skin, bone and othertissues. The AC component is caused by the time varying absorption oflight caused by temporal changes in blood volume below the sensor.

Changes in the blood volume can be caused by cardiovascular regulation,blood pressure regulation, thermoregulation and respiration. Thus theplethysmogram can be analysed to determine information on suchparameters as pulse rate, breathing rate, blood pressure, perfusion,cardiac stroke volume and respiratory tidal volume. These can beobserved as periodic and non-periodic changes in the amplitude of AC andDC components in the plethysmogram. This has been described in moredetail in Kamal et al: ‘Skin Photoplethysmography—a review’, ComputerMethods and Programs in Biomedicine, 28 (1989) 257-269). Theplethysmogram can also be analysed to determine blood constituents. Onesuch technique is pulse oximetry, which determines the relative amountof oxygen in the blood. Other blood constituents can also be measured byusing photoplethysmography.

There are two modes of photoplethysmography, the transmission mode andthe reflection mode. In transmission mode the light source is on oneside of the tissue and the photodetector is placed on the other side,opposite the light source. The use of transmission mode is limited toareas where the tissue is thin enough to allow light to propagate, forexample the fingers, toes and earlobes of a human subject.

In reflection mode the light source and photodetector are placeside-by-side. Light entering the tissue is reflected and a proportion ofthis is detected at the photodetector. This source-detectorconfiguration is more versatile and allows measurements to be performedon almost any area of tissue. However, the use of reflectance mode ismuch harder to design than transmission because the signal level issignificantly lower at the most effective wavelengths. Thus,considerable attention must be given to maximising signal-to-noiseratio. As a result, the most common PPG sensors use transmission modeand hence are restricted to positions where light can pass throughtissue.

As a photodetector is used to measure light from the source, thephotoplethysmograph can also respond to interfering signals from othersources of light, for example fluorescent lighting and computermonitors. The sensor must also respond to changes in the lightpropagating through tissue, i.e. the plethysmogram. These physiologicalchanges contain frequency components between DC and 25 Hz. However, itis desirable for the sensor not to respond to ambient light noise.Accordingly, the photoplethysmograph should reject ambient light noisewhile detecting the plethysmogram in the bandwidth of interest.

A second source of interference is other electrical apparatus. Otherelectrical devices can generate radio frequency signals that aphotoplethysmograph can detect. It is desirable to minimise thesensitivity of the system to interfering sources of this nature.

A third source of interference is the electrical noise generated by thephotoplethysmograph itself. Such noise can be generated by electroniccomponents, and can include thermal noise, flicker noise, shot noise, aswell as noise spikes, for example, harmonics generated by missing codesin an analogue-to-digital converter. It is also desirable to minimisethe sensitivity of the system to interference from these sources.

A known technique for reducing the noise generated by these threesources of interference is to drive the sensor's light source with acarrier modulated at a frequency that is not present, or dominant, inthe ambient light, electrical radio frequency signals, orphotoplethysmograph system noise. This can be done by modulating thesensor's light source with a square wave, by pulsing it on and off. Thedetected signals are then band pass filtered to attenuate interferenceoutside the frequency range of interest. Subsequent demodulation willrecover the plethysmogram. In general, any periodic signal such as asine wave may be used to modulate the light source.

Though modulated light photoplethysmography exists in the prior art,there are still critical limitations in how it has been applied,especially in terms of suitable signal conditioning circuits forattenuating or removing noise, and demodulation. For example, EP0335357,EP0314324, WO0144780 and WO9846125 disclose modulated lightphotoplethysmography. However, they use a demodulation method andapparatus that requires the modulating and demodulating carrier phase tobe synchronised. Error in the synchronisation timing will add noise tothe demodulated signal (timing jitter or phase noise). The prior artalso fails to make full use of band pass filter characteristics toremove ambient interfering light, by still relying on a separate channelto measure ambient light, and later subtracting it from the signal,which adds further complexity and is arguably less efficient atattenuating interference. These limitations reduce immunity to broadbandand narrowband noise from sources such as fluorescent lighting, computermonitors, sunlight, incandescent light, electrical RF interference,thermal noise, flicker noise, and shot noise.

A further limitation in the prior art is the choice of wavelength forreflectance mode sensors. Both reflection mode and transmission modesensors use light sources in the red and/or infrared part of thespectrum, wavelengths between 600 nm and 1000 nm being typical. However,red/infrared reflectance sensors do not function well because light atred and infrared wavelengths is poorly absorbed by blood. This resultsin low modulation of the reflected signal and therefore a small ACcomponent. Therefore red/infrared reflectance probes give poor resultswhen compared to transmittance probes. It has been shown in Weija Cui etal: “In Vivo Reflectance of Blood and Tissue as a Function of LightWavelength”, IEEE Transactions on Biomedical Engineering, Volume 37, No6, June 1996), that a larger plethysmogram AC component amplitude can berecorded if a reflectance mode sensor uses light of wavelengths between500 nm and 600 nm (green light).

A continuous non-modulated green light photoplethysmograph was describedin WO 9822018A1. However, the objective of this invention wasreflectance pulse oximetry, and the patent does not explain the stepsnecessary to produce a reliable photoplethysmograph suitable formeasuring the plethysmogram AC and DC component. Such a green lightsensor would be necessary to reliably detect the AC component, forexample heart rate, but moreover the breathing signal, which isextremely small and was not detected by this system.

In Benten et al: “Integrated synchronous receiver channel for opticalinstrumentation applications” Proceedings of SPIE—The InternationalSociety for Optical Engineering, Volume 3100, 75-88, 1997), a modulatedlight reflectance photoplethysmograph is described that uses a switchingmultiplier to systematically change the gain of the signal path between+1 and −1. This is the equivalent of mixing the modulated signal with asquare wave to recover the plethysmogram. However, similar to the otherprior art described previously, this method needs the modulating carrierand demodulating local oscillator signals to be in-phase.

It is an object of the present invention to provide an improvedplethysmograph.

According to one aspect, the present invention provides aphotoplethysmograph device comprising:

-   -   a light source for illuminating a target object;    -   a modulator for driving the light source such that the output        intensity varies as a function of a modulation signal at a        modulation frequency;    -   a detector for receiving light from the target object and        generating an electrical output as a function of the intensity        of received light;    -   a demodulator for receiving the detector output, having a local        oscillator and producing a demodulated output representative of        the modulation signal and any sidebands thereof, in which the        demodulator is insensitive to any phase difference between the        modulation signal and the oscillator of the demodulator; and    -   means for generating, from the demodulated output, a signal        indicative of blood volume as a function of time and/or blood        composition.

According to another aspect, the present invention provides a method ofgenerating a plethysmogram, comprising the steps of:

-   -   illuminating a target object with a light source;    -   driving the light source with a modulator such that the output        intensity varies as a function of a modulation signal at a        modulation frequency;    -   receiving light from the target object with a detector and        generating an electrical output as a function of the intensity        of received light;    -   receiving the detector output in a demodulator having a local        oscillator and producing a demodulated output representative of        the modulation signal and any sidebands thereof, in which the        demodulator is insensitive to any phase difference between the        modulation signal and the oscillator of the demodulator; and    -   generating, from the demodulated output, a signal indicative of        blood volume as a function of time and/or blood composition.

According to another aspect, the present invention provides aphotoplethysmograph device comprising:

-   -   one or more light sources each for illuminating a portion of a        target object;    -   one or more modulators for driving the light sources such that        the output intensity of each light source varies as a function        of a modulation signal at a modulation frequency;    -   one or more detectors for receiving light from the target object        and generating one or more electrical outputs as a function of        the intensity of received light;    -   a plurality of demodulators each for receiving one or more of        the electrical outputs and producing a demodulated output        representative of the modulation signal of one of the modulated        light sources and any sidebands thereof, to thereby produce a        plurality of demodulated outputs corresponding to the plurality        of light sources and/or plurality of detectors; and    -   means for generating, from the demodulated outputs,        plethysmogram signals indicative of blood volume as a function        of time and/or blood composition for each of the demodulator        outputs.

According to another aspect, the present invention provides a method ofgenerating a plethysmogram, comprising the steps of:

-   -   illuminating a portion of a target object with one or more light        sources;    -   driving the light sources with one or more modulators such that        the output intensity of each light source varies as a function        of a modulation signal at a modulation frequency;    -   receiving light from the target object with one or more        detectors and generating one or more electrical outputs as a        function of the intensity of received light;    -   receiving one or more of the electrical outputs with a plurality        of demodulators, each producing a demodulated output        representative of the modulation signal of one of the modulated        light sources and any sidebands thereof, to thereby produce a        plurality of demodulated outputs corresponding to the plurality        of light sources; and    -   generating, from the demodulated outputs, plethysmogram signals        indicative of blood volume as a function of time and/or blood        composition for each of the demodulator outputs of the pixel        array.

According to another aspect, the present invention provides aphotoplethysmograph device for non-contact use, comprising:

-   -   a light source for illuminating a target object via a first        polarising filter;    -   a modulator for driving the light source such that the output        intensity varies as a function of a modulation signal at a        modulation frequency;    -   a detector for receiving light from the target object via a        second polarising filter having a different polarisation state        than the first polarising filter, the detector adapted to        generate an electrical output as a function of the intensity of        received light;    -   a demodulator for receiving the detector output and producing a        demodulated output representative of the modulation signal and        any sidebands thereof; and    -   means for generating, from the demodulated output, a signal        indicative of blood volume as a function of time and/or blood        composition.

According to another aspect, the present invention provides a method ofgenerating a photoplethysmogram, comprising the steps of:

-   -   illuminating a target object with a light source via a first        polarising filter;    -   driving the light source with a modulator such that the output        intensity varies as a function of a modulation signal at a        modulation frequency;    -   receiving light from the target object with a detector via a        second polarising filter having a different polarisation state        than the first polarising filter, the detector generating an        electrical output as a function of the intensity of received        light;    -   receiving the detector output with a demodulator and producing a        demodulated output representative of the modulation signal and        any sidebands thereof; and    -   generating, from the demodulated output, a signal indicative of        blood volume as a function of time and/or blood composition.

According to another aspect, the present invention provides aphotoplethysmograph device for non-contact use, comprising:

-   -   a light source for illuminating a target object with optical        radiation of wavelength less than 600 nm;    -   a modulator for driving the light source such that the output        intensity varies as a function of a modulation signal at a        modulation frequency;    -   a detector for receiving light from the target object and        adapted to generate an electrical output as a function of the        intensity of received light, the light source and detector being        disposed laterally adjacent to one another on a substrate such        that the active surfaces thereof can be directed towards        substantially the same point on a surface of the target body;    -   a demodulator for receiving the detector output and producing a        demodulated output representative of the modulation signal and        any sidebands thereof; and    -   means for generating, from the demodulated output, a signal        indicative of blood volume as a function of time and/or blood        composition.

According to another aspect, the present invention provides a method ofgenerating a photoplethysmogram, comprising the steps of:

-   -   illuminating a target object with optical radiation of        wavelength less than 600 nm from a light source;    -   driving the light source with a modulator such that the output        intensity varies as a function of a modulation signal at a        modulation frequency;    -   receiving light from the target object with a detector to        generate an electrical output as a function of the intensity of        received light, the light source and detector being disposed        laterally adjacent to one another on a substrate such that the        active surfaces thereof can be directed towards substantially        the same point on a surface of the target body;    -   receiving the detector output with a demodulator and producing a        demodulated output representative of the modulation signal and        any sidebands thereof; and    -   generating, from the demodulated output, a signal indicative of        blood volume as a function of time and/or blood composition.

The invention provides a modulated light photoplethysmograph device. Inselected embodiments, it combines the features of modulated light, bandpass filtering, and IQ demodulation to give a plethysmogram of perfusetissue. When used in reflectance mode, light in the blue and/or greenportion of the optical spectrum is used which gives a larger pulsatilesignal and improved signal to noise ratio.

Selected embodiments of the invention provide improved reliabilitythrough the reduction of noise when the photoplethysmograph device isused in transmission mode. In addition, the choice of light in theblue/green portion of the optical spectrum (i.e. wavelengths of between400 nm and 600 nm) gives improved reliability through the reduction ofnoise and the increase in AC component signal amplitude, when thephotoplethysmograph device is used in reflection mode.

Selected embodiments can be applied to different photoplethysmographytechniques including single wavelength photoplethysmography, multiplewavelength photoplethysmography, pixel array photoplethysmography, andnon-contact photoplethysmography.

Embodiments of the present invention will now be described by way ofexample and with reference to the accompanying drawings in which:

FIG. 1 is a functional block diagram of a single wavelengthphotoplethysmograph device;

FIG. 2 is a functional block diagram of a demodulator suitable for usein the photoplethysmograph device of FIG. 1;

FIG. 3 is a functional block diagram of a multiple wavelengthphotoplethysmograph device;

FIG. 4 is a schematic plan view of a pixel array photoplethysmographdevice;

FIG. 5 a is a schematic side view of a non-contact photoplethysmographdevice with polarising filters;

FIG. 5 b is a plan view of a polarising filter for use with thereflectance mode photoplethysmograph device of FIG. 7;

FIG. 6 is a functional block diagram of a single wavelengthphotoplethysmograph device;

FIG. 7 is a schematic plan view, side view and end view of a reflectancemode photoplethysmograph device;

FIG. 8 is a circuit diagram of a transimpedance amplifier suitable foruse in the photoplethysmograph devices described herein;

FIG. 9 is a circuit diagram of a band pass filter circuit suitable foruse in the photoplethysmograph devices described herein;

FIG. 10 is a circuit diagram of a light source driver circuit suitablefor use in the photoplethysmograph devices described herein;

FIG. 11 is a process flow diagram illustrating a demodulation algorithmsuitable for use in the photoplethysmograph devices described herein;

FIG. 12 is a functional block diagram of a light source brightnesscontrol loop suitable for use in the photoplethysmograph devicesdescribed herein;

FIG. 13 a is a photoplethysmogram showing a combined AC and DC output ofa photoplethysmograph device;

FIG. 13 b is a photoplethysmogram showing the magnified AC componentfrom FIG. 13 a;

FIG. 14 a is a photoplethysmogram showing combined pulsatile andbreathing signal;

FIG. 14 b is a photoplethysmogram showing the breathing signal of FIG.14 a only;

FIG. 15 a is a photoplethysmogram showing a breathing signal only;

FIG. 15 b is a corresponding breathing signal as measured by an oralthermistor;

FIG. 16 is a photoplethysmogram recorded using a green light source ofwavelength 510 nm;

FIG. 17 is a photoplethysmogram recorded using a red light source ofwavelength 644 nm;

FIG. 18 is a functional block diagram of an alternative demodulatorsuitable for use in the photoplethysmograph device of FIG. 1;

FIG. 19 is a functional block diagram of an alternative demodulatorsuitable for use in the photoplethysmograph device of FIG. 1;

FIG. 20 is a functional block diagram of an alternative demodulatorsuitable for use in the photoplethysmograph device of FIG. 1.

SINGLE WAVELENGTH PHOTOPLETHYSMOGRAPH DEVICE

With reference to FIG. 1, a photoplethysmograph device 100 comprises adriver circuit 101 which is coupled to energise a light source 102 withmodulated drive signal such that the output intensity of the lightsource varies as a function of a modulation signal having a specificmodulation frequency (f_(m)) and modulation amplitude (M1(t)). Thewaveform driving the light source is therefore a modulating carriercharacterised by its frequency and amplitude.

The light source 102 is configured to illuminate a target object 103such as an area of tissue of the human or animal body. The light source102 preferably comprises one or more light emitting devices each of agiven wavelength or range of wavelengths.

A photodetector 104 is configured to receive light from the targetobject 103 after its interaction therewith. Depending on the relativepositioning of the light source 102, the target object 103 and thephotodetector 104, this received light may be one or more of light thathas been transmitted through the target object, light that has beenreflected from the surface of the target object, and light that has beenscattered by and/or reflected from structures or fluids within thetarget object. The photodetector will generate an electrical currentthat is a function of, e.g. proportional to, the amount of lightincident to its active area.

A detector 105 may be provided to convert the electrical current fromthe photodetector 104 to a voltage that is proportional to the current.The detector 105 may incorporate an amplifier (not shown). The gain ofthat amplifier can be rolled off at a frequency greater than themodulation frequency. The detector 105 and amplifier can, with carefuldesign, minimise the noise at the input to a band pass filter 106coupled thereto. In a general sense, the photodetector 104 and detector105 functions may be provided by any detector capable of receiving lightfrom the target object and generating an electrical output that is afunction of the intensity of the received light.

The band pass filter 106 may be provided for attenuating signals outsidea bandwidth of interest. The filter bandwidth is preferably centred onthe modulation frequency f_(m) and is sufficiently wide to pass themodulating carrier and sidebands caused by plethysmogram amplitudemodulation, but narrow enough to attenuate frequency components ofinterference and noise. To reduce noise, the bandwidth of the band passfilter 106 should be as narrow as possible. It need only be wide enoughto pass the upper and lower sidebands of the plethysmogram, typicallybut not limited to 50 Hz. The band pass filter 106 may incorporate anamplifier (not shown) to provide additional gain. The band pass filter106 and amplifier are preferably designed to minimise noise at the inputof the following stage, namely a demodulator 107. It will be appreciatedthat the provision of a band pass filter 106 is not always necessarybut, if employed, an increase in signal-to-noise ratio (SNR) may result.

A preferred arrangement of demodulator 107 is shown in more detail inFIG. 2. The demodulator 107 is adapted to demodulate the output of theband pass filter 106 and hence recover a plethysmogram from the detectedlight received from the target object. The preferred demodulator 107uses a method that is insensitive to the phase difference between themodulation carrier and a demodulation carrier. In other words, thedemodulator is insensitive to any phase difference between themodulation signal and an oscillator in the demodulator, as will beexplained later. Thus, it is unnecessary to maintain a predeterminedphase relationship between the modulation and demodulation process.

The demodulator 107 may comprise a multiplexer 210 for splitting themodulated signal M1(t) into two channels. A first channel processes afirst modulated input signal M1(t)a and a second channel processes asecond modulated input signal M1(t)b. The first modulated input signalM1(t)a is provided as input to a first multiplier 201 together with anoutput of a first demodulator local oscillator (LO) signal 204, D1(t).The frequency of the local oscillator signal 204 is preferablysubstantially equal to the frequency of the modulation signal andtherefore equal to the modulating carrier frequency of input signalM1(t). The result of the multiplication of M1(t)a with the first LOsignal 204 is an I (‘in phase’) signal. In the second channel, thesecond modulated input signal is multiplied, using a multiplier 206,with a second demodulator local oscillator (LO) signal that also has afrequency preferably substantially equal to the frequency of themodulation signal. However, the second demodulator LO signal is phaseshifted by phase shifter 205 with respect to the first demodulator LOsignal. The phase difference between the first demodulator LO and seconddemodulator LO is preferably 90 degrees. The result of themultiplication of M1(t)b with the second demodulator LO signal is the Q(‘quadrature phase’) signal. It will be understood that the localoscillator, although shown as a producing a sine wave output, couldproduce other waveforms of the required frequency.

The separate I and Q signals are preferably separately low pass filteredin filter elements 202 and 207 respectively to remove unwanted harmonicsand products of the multiplication process. Optionally, the resultingsignals may be decimated in decimators 203 and 208 respectively toreduce the sample rate. The results of this are the I′ and Q′ signals.

The I′ and Q′ signals can be demultiplexed back into one signal at mixer209 to provide the demodulated plethysmogram S1(t). The demultixplexingprocess can include an algorithm or circuit that determines the squareroot of the sum of the squares of the I′ and Q′ signals.

The demodulator arrangement of FIG. 2 can be modified while stillproviding a demodulator that is insensitive to any phase differencebetween the modulation signal and the oscillator in the demodulator.FIGS. 18 to 20 show alternative arrangements each providing two channelsin which, in the first channel the detector output is mixed with a localoscillator having a first phase relationship with the detector outputand in the second channel the detector output is mixed with a localoscillator having a second phase relationship with the detector output.As in FIG. 2, the first and second phase relationships are preferably 90degrees apart.

From inspection of the figures, it will be seen that this can beachieved by using a common local oscillator 204, 1804, 1904, 2004 thatfeeds the two channels with a different relative phase shift element205, 1905 a, 1905 b, 2005 (FIGS. 2, 19 and 20) or by using a commonlocal oscillator but a phase delay element 1805 (FIG. 18) provided inone or both channels to delay one or both of the signals M1(t)a andM1(t)b and thereby create a relative phase shift between them. FIG. 20also illustrates that the filtering otherwise carried out by elements202, 207 (or 1802, 1807, 1902, 1907) can alternatively be carried outafter the mixer 209, 1809, 1909, 2009 by a filter 2002. Similarly,decimation can also be carried out after the mixer 209, 1809, 1909,2009.

A means of detecting and attenuating harmonically related narrow bandnoise may be provided. This means can be adaptive so that changes ininterference characteristics can be detected and filtering (or othermeans of rejection) adapted to maintain signal-to-noise ratio.

A means of closed loop control can be provided to maintain the lightsource 102 at a brightness sufficient to detect the plethysmogram. Afunctional block diagram of this control loop 1200 is shown in FIG. 12.Similar elements to that shown in FIG. 1 are given correspondingreference numerals. The amplitude of the detected, band pass filtered,modulation carrier D-BPF-M1(t) can be measured and processed by a signalconditioning circuit 1201 in the feedback path, then compared with areference value or range of values in comparator 1202. An error signalcan then be generated and processed by a signal conditioning circuit oralgorithm 1203 in the forward path. By using this technique, theamplitude of the waveform generated by the driver circuit can beadjusted to ensure the detected carrier amplitude falls within the givenrange, or near the reference. This will ensure, for example, that if toomuch light is received from the target object, the detector does notsaturate, or if too little light is received from the target object 103,the plethysmogram does not go undetected. Thus, in a general aspect, thefeedback control loop 1200 provides an example of a means formaintaining the output intensity of the light source 102 as a functionof the detector output and at a level adequate to maintain detection ofa plethysmogram from the demodulated output S1(t).

Multiple Wavelength Photoplethysmography

FIG. 3 illustrates a photoplethysmograph device 300 that includes two ormore light sources 302, 304 for emitting light at two or more differentwavelengths into the target object (e.g. a tissue under test). Anoptical detector 306 is adapted to detect the light received from thetarget object, e.g. transmitted through the target object when thephotoplethysmograph device is in transmission mode or reflected from thetarget object when the photoplethysmograph device is used in reflectionmode. Driver circuits 301, 303 respectively are provided to energise thelight sources 302, 304 each with a modulated drive signal having amodulation at a selected frequency and amplitude M1(t) and M2(t). Onlytwo drivers and light sources are illustrated but it will be understoodthat generally a plurality of drivers and light sources can be used.Each light source can consist of one or more optical emitters that emitlight at a single, given wavelength or range of wavelengths. Thewaveform of each light source can have a frequency different from thoseused to energise the other light sources. This waveform is themodulating carrier and is characterised by its frequency and amplitude.Each light source can optionally have a separate associated drivercircuit. Each light source can optionally have a different wavelength.

A photodetector 306 is provided to detect light after its interactionwith the target object 305 (e.g. tissue of a human or animal body). Thephotodetector 306 will generate a current proportional to the amount oflight incident to its active area.

A detector 307 may be provided to convert the current from thephotodetector 306 to a voltage that is proportional to the current. Thedetector 307 can incorporate an amplifier (not shown). The gain of thatamplifier can be rolled-off at a frequency greater than the highestmodulation frequency. The detector and amplifier can, with carefuldesign, minimise the noise at the input to a band pass filter 308 andhence maximise the signal-to-noise ratio.

The band pass filter 308 may be provided for attenuating signals outsidea bandwidth of interest. The filter bandwidth is preferably chosen sothat the filter's lower roll-off is below the lowest modulating carrierfrequency and the filter's upper roll-off is above the highestmodulating carrier frequency. The bandwidth between the highest andlowest modulating carrier frequencies and the filter roll-off should besufficiently wide to pass the modulating carrier and sidebands caused byplethysmogram amplitude modulation, but narrow enough to attenuatefrequency components of interference and noise. To reduce noise thefilter bandwidth should be as narrow as possible. It need only havesufficient range to pass the upper sideband of the highest modulationcarrier and the lower sideband of the lowest modulation carrier.Typically 25 Hz above the highest modulating carrier frequency to 25 Hzbelow the lowest modulating carrier frequency is adequate. The band passfilter can incorporate an amplifier (not shown), which would provideadditional gain. The band pass filter and amplifier can be designed tominimise noise at the input of the following stage. Filters can beincluded that provide a frequency response with a null or largeattenuation at multiples of a fundamental frequency, for example a combor moving average filter. These filters can be designed to attenuate thefundamental and harmonics of an interfering source.

Multiple demodulators 309 and 310 are provided for demodulating theoutput of the band pass filter to recover the plethysmogram at eachmodulating carrier frequency or at each wavelength of light. Preferably,the demodulators use a method of demodulation that is insensitive to thephase difference between each the modulating carrier and demodulatinglocal oscillator, such as that described in connection with FIG. 2.Therefore, as previously stated, it is unnecessary to maintain apredetermined phase relationship between the modulation and demodulationprocess.

In this case, each demodulator will have a local oscillator D1(t) andD2(t) that preferably have the same frequency as the correspondingmodulating carrier M1(t) and M2(t) respectively.

The output of this multiple wavelength photoplethysmograph device ismultiple plethysmograms S1(t) and S2(t). Each is the plethysmogram for agiven wavelength of light used to test the tissue. It will be understoodthat though the multi wavelength photoplethysmograph has been describedwith an example of two wavelengths, one wavelength provided by lightsource 302 and the second wavelength provided by light source 303, theinvention can be modified to use more than two wavelengths by addingadditional drivers, light sources and demodulators. These modulatedmultiple wavelengths not only allow selection of optical wavelengths foroptimum SNR for the detection of pulse and breathing rate but also allowratiometric measurements to be carried out to determine bloodconstituents. Thus, in a general aspect, the photoplethysmograph devicemay provide a means for automatically selecting one of the demodulatedoutputs S1(t), S2(t) that provides the best SNR for the data to beextracted from the plethysmogram.

As described above in connection with FIG. 12, closed loop control canalso be used to maintain each light source at a given brightness.

Pixel Array Photoplethysmography

A combination of photodetector, detector, band pass filter anddemodulator may be used to form one pixel of a multi-pixelphotoplethysmograph imaging apparatus. Such an array can be produced asa microchip with the pixel and analogue or digital signal processingperformed on chip.

FIG. 4 shows a schematic plan view of a small (4×4) pixel arrayphotoplethysmograph device 400, comprising sixteen pixels 401. It willbe understood that the array can be considerably larger than this ifrequired.

Each pixel 401 preferably comprises a photodetector, detector circuit,band pass filter, and demodulator. Such an apparatus provides sixteensimultaneous (parallel) plethysmograms, detected by light from tissueilluminating each pixel in the array. The array does not have to besquare. For example, the array could comprise 4×16 pixels, or 1×256pixels etc. Each pixel may respond to light from a common light sourcemodulated with a common modulation frequency. Alternatively, each pixelcould correspond to a respective independently driven light source, sothat different modulation frequencies could be used for each pixel.Alternatively, each pixel could correspond to a respective light sourcewith all light sources being driven using a common modulation signal.

An array of detectors opens up a whole new dimension of signalprocessing by using several parallel channels of the aforementionedprocessing. The pixel array enables the production of a spatial map ofblood parameters (e.g. pulse rate, breathing rate and bloodconstituents) from the target object. Multiple channels may be processedin parallel thereby allowing an arbitration scheme to be employed toselect the optimum SNR. Further the multiple channels can be processedby independent component analysis, principal component analysis or blindsource separation for example, to extract the fundamental signal whenburied within noise and other interfering signals. It is therebypossible to produce robust pulse and breathing rate measurements andspatial blood constituent measurements when more than one wavelength isused. Independent component analysis etc can also be used to reducemovement artefacts. Movement artefact is often a serious problem forphotoplethysmograph systems: the problem and other methods of reductionhave been described by Smith and Hayes (Matthew J. Hayes and Peter R.Smith, “Artifact reduction in photoplethysmography”. Applied Optics,Vol. 37, No. 31, November 1998).

A signal processing means implemented on- or off-chip in an analogue ordigital domain that analyses the plethysmogram from each pixel may beimplemented to extract the breathing rate, pulse rate, bloodconstituents etc. In general both pre- and post-processing can beperformed for each pixel thereby allowing full field, spatial signalprocessing algorithms to be used.

Non-Contact Photoplethysmography

The single wavelength photoplethysmograph devices, multiple wavelengthphotoplethysmograph devices, and pixel array photoplethysmograph devicesdescribed above can each be used in a non-contact reflection mode.

In photoplethysmography the photodetector 104 is in contact with thetarget object, e.g. the tissue surface. A large proportion of the lightfrom the source 102 is reflected from the tissue surface, but becausethe photodetector 104 is in contact with the tissue, this surfacereflected light is not detected. A small proportion of light penetratesand interacts with the tissue, and then emerges incident to thephotodetector, where it is detected, amplified and processed, whichgives rise to the plethysmogram signal.

In non-contact photoplethysmography the photodetector 104 is not incontact with the tissue. This results in detection of the greaterproportion of light reflected at the tissue surface as well as lightthat has penetrated the tissue. The detected signal now comprises a muchlarger DC offset caused by the reflected light, onto which issuperimposed a much smaller plethysmogram signal. The light reflectedfrom the tissue surface has not interacted with blood and hence containsno information useful to the plethysmogram.

It will be appreciated that the DC offset caused by tissue surfacereflections will reduce the dynamic range of the photoplethysmograph.Therefore it is of benefit to filter out reflected light when using anon-contact photoplethysmograph. This can be done by using a polarisingfilter.

A polarising filter selectively polarises or filters light polarisedalong a given axis. This polarity is retained when light is reflectedbut lost when light is scattered. If the light incident on the tissue ispolarised, the light reflected at the surface retains this polarity andcan be attenuated by a filter orientated with its polarity at 90 degreesto that of the incident light. However, light that penetrates the tissueand is scattered by the blood and other media loses its polarity andhence passes through the horizontal polarising filter, and is detectedby the photodetector.

With reference to FIG. 5, a first polarising filter 504 polarises lightfrom a modulated light source 501 along a given polarisation axis P1.The polarised light is directed towards the target object 503, fromwhich a proportion of the light is reflected from the surface andscattered from within the target object.

A second polarising filter 505 is disposed in front of the detector 502that receives the light from the target object. The second polarisingfilter has a polarisation axis P2 and attenuates polarised lightincident to the photodetector. The attenuation is at its greatest whenthe polarisation axis P2 of the second polarising filter 505 is at 90degrees (orthogonal) to the axis of the polarised light. Thus, the firstand second polarising filters 504, 505 are preferably arranged so thattheir respective polarisation axes P1, P2 are orthogonal to one another.In this way, the light reflected from the surface that retains itspolarisation is substantially or completely attenuated, while the lightthat has been scattered from media within the target object and has lostits polarisation state has significantly reduced attenuation.

The devices described above can provide significant attenuation ofnarrow band interference that result from sources of ambient light (suchas those produced by fluorescent lamps, computer monitors andincandescent light bulbs), electromagnetic interference, and noisespikes intrinsic to the apparatus and method of plethysmography, forexample harmonics generated in analogue-to-digital converters. Themodulation and demodulation frequencies may be selected to avoidharmonics of these interferences and, in conjunction with filtering, toattenuate broadband noise intrinsic to the device including white noise,flicker noise and shot noise.

In the arrangement of FIG. 2, it is not necessary to know or maintainthe phase relationship between the modulating carrier and demodulatinglocal oscillator, because the demodulation process is insensitive to thephase difference between the two. It is therefore not necessary tocalibrate for or consider any constant phase delay in the detectedsignal caused by the signal conditioning circuitry or the propagation oflight in tissue.

It will also be appreciated that the technical features can be embodiedin various forms. For example, the driver circuit, light source,photodetector, detector, band pass filter and demodulation process canbe implemented, where appropriate, as a digital signal processingalgorithm, a custom analogue integrated circuit, discrete analogueelectronic components or as a combination of analogue and digital signalprocessing functions.

A further modification would be to sample the output of the detectorcircuit and implement band pass filter, demodulator and signalprocessing on a digital signal processor or microprocessor as part of asignal processing algorithm.

A further modification would be to implement the photodetector,detector, band pass filter, demodulator and signal processor on amicrochip as a VLSI mixed signal design.

Variants and Noise Management

A combination of some or all of each of these features can be utilisedto produce the desired system such that the signal can be separated fromthe noise. However, careful design should be made at the delivery of thelight source and the collection of the received light signal. Forexample the magnitude of the photoplethysmogram current detected in thetotal photocurrent is quite small and hence a poorly designed front endcan result in a distorted or submerged signal in amongst the noise. Thedelivery of a pulsed voltage to the light source should be made bycabling that is shielded and does not run alongside the receivingphotodiode connections. If this occurs then a displacement current maybe induced in the photodetector equal to I=CdV/dt.

Depending upon the magnitude of the light source power, the detectorsize and the rate of change of voltage will establish the maximum valueof the coupling capacitance allowed. It is good design practice toensure that the induced displacement current is limited to no more thantypically 1% of the detected current.

Other design criteria may be as follows:

-   -   a) The input bias current should be less than ˜1% of the DC        light level detected.    -   b) The voltage and current noise should be less than the shot        noise set by the DC light level detected.    -   c) The choice of a transimpedance amplifier should be made such        that its 1/f corner frequency is less than the modulating        carrier frequency.    -   d) The carrier rise and fall time can be slewed to reduce the        coupling.    -   e) Good PCB design practice can be observed to avoid the        coupling of signals from high power noisy components to a        sensitive sensor front end, particularly a transimpedance        amplifier. Multilayer PCBs can be used to keep power supplies        and ground returns as short as possible and therefore minimise        ground bounce and other forms of noise coupling. Multilayer PCB        design can be used for a reflectance probe to reduce the        coupling of displacement currents from the light source voltage        pulse to the receiving photo diode connection.

There now follows an example configuration. However, it should be notedthat this is not the only configuration as combinations of some or allof these features can lead to a beneficial design.

EXAMPLE

FIG. 6 is a functional block diagram illustrating the architecture of apreferred plethysmography device 600 including a driver circuit 601 fordriving a light source 602 with a modulated carrier signal such that theoutput intensity varies as a function of the modulation signal at amodulation frequency. The light source illuminates a target object 603and light returned from the target object is received by photodetector604 to generate an electrical signal as a function of the intensity ofreceived light. Detector 605 converts the electrical current output ofphotodetector 604 to a voltage signal. This is filtered by bandpassfilter 606 and converted to a digital signal in analogue to digitalconverter 607. A demodulator 608 (which may be of the type described inconnection with FIG. 2) has a local oscillator signal D1(t) which ispreferably substantially the same frequency as the modulation signalM1(t) of driver circuit 601. A block average filter 609 is used toproduce an output plethysmogram S1(t).

FIG. 7 illustrates a reflectance probe 700 providing the light sourceand photodetectors for the apparatus of FIG. 6. The reflectance probe700 comprises four light emitting devices 702 for emitting modulatedlight signals of a single wavelength, in order to illuminate the tissueunder test. A photodiode 704, which could be an array of detectors, hasa given active area 703 which is used to detect the light reflected backfrom the tissue under test. A suitable polarising filter element 510incorporating first and second crossed polarising filter elements 511and 512 is shown in FIG. 5 b. When the probe is used in non-contactmode, this element 510 is placed over the top of light emitting devices702 and photodiode 704. In a general aspect, this arrangement providesactive surfaces of a light source and detector directed towardssubstantially the same point on a surface of a target body.

The light emitting devices are preferably light emitting diodes (LEDs)with a peak spectral response between 400 nm and 600 nm. Generally, thewavelength is selected based on the optical characteristics of thetissue under investigation. This exemplary photoplethysmograph device700 is particularly suited to the measurement of heart rate andbreathing rate in humans, therefore the wavelengths are selected basedon the optical properties of human tissue and blood which exhibit strongabsorption characteristics between 400 nm and 600 nm. Studies weremainly carried out at the absorption spectrum between 500 nm and 600 nm.However a strong absorption spectrum also exists at 440 nm and a deviceoperating around this wavelength would also produce favourable results.More specifically, there are three versions of the reflectance probe:one with LEDs that have a peak spectral response of 512 nm; one withLEDs that have a peak spectral response of 562 nm; one with LEDs thathave a peak spectral response of 574 nm. These are the preferredwavelengths because they are commercially available and economicalhowever others can be used if supply and economics permit. The range ofwavelengths between 500 nm and 600 nm is particularly preferred since,although the signal may improve below 500 nm, the penetration depth ofthe light decreases which may, in some circumstances, result ininsufficient light reaching the pulsatile blood in the skin'sarterioles.

The LEDs and photodiode are mounted side by side on four-layer printedcircuit board (PCB). The use of screened power and signal cables andmulti-layer PCB design improves immunity to noise pickup and electricalcross talk. The height of the photodiode package is preferably greaterthan that of the LEDs to reduce direct coupling of light onto the activearea (optical crosstalk). The lateral separation between the LEDs 702and photodiode active area 703 increases the path length that the lightmust travel through the tissue which improves the signal.

The light source is excited at a given frequency and amplitude by amodulating carrier from the driver circuit 601. The driver circuit 601is a digital-to-analogue converter implemented using a current summingamplifier as shown in FIG. 10. An 8-bit DAC input signal is generated bya microcontroller and presents 255 discrete amplitude levels viaresistors 1001. The carrier frequency is determined by the rate at whichthe input signal is clocked. The output signal M(t) is a square wave ofa given carrier frequency with an amplitude that can be varied between 0volts and the fall-scale output range of the op amp 1002. The closedloop voltage gain of the op amp 1002 is set by the inverting feedbackfraction of resistors 1003 and 1004. This can be adjusted so that adigital input of 255 gives a full-scale analogue output.

Light incident to the photodiode 604 may be passed through a visiblelight optical filter that attenuates wavelengths above 600 nm. Thefilter may be incorporated into the photodiode and positioned in frontof its active area. The peak spectral response of the photodiode ispreferably between 500 nm and 600 nm. More specifically, the peakspectral response of the photodiode may be 580 nm. The light filter mayroll off the photodiode response above 600 nm which serves to attenuateinterference from light above this wavelength.

Light incident on the photodiode 604 generates an analogue current. Thephotodiode current is coupled to the current-to-voltage converter 605,which may be a transimpedance amplifier 800 as illustrated in FIG. 8.The transimpedance amplifier 800 is preferably designed so that its gainrolls off above the modulation frequency. This low pass filter responsereduces noise and aliasing. The amplifier 800 is designed so that afeedback capacitor 801 is as near as possible to the value of thephotodiode junction capacitance which reduces voltage noise gain. Thismust be balanced against the requirement for transimpedance roll-off andamplifier stability which is controlled by the feedback capacitor 801and resistor 802.

The band pass filter 606 is preferably an active Sallen-Key type with anRC frequency response though it will be appreciated that Chebychev,Butterworth and other responses could be used. An exemplary filter 900is shown in more detail in FIG. 9. Though the filter 900 is designedusing an operational amplifier 906, it will be appreciated that a bandpass filter frequency response can be produced by other methods. Thefilter 900 is designed to have a centre frequency as close as possibleto the modulating frequency which, in this example, is 570 Hz, and lowtolerance components are selected to help achieve this. The invertinginput feedback network 907 and 908 of the operational amplifier sets thefilter gain and bandwidth. This is preferably designed to give as narrowas possible bandwidth while not making the filter centre frequencyoverly sensitive to tolerance of components 901, 902, 903, 904 and 905.The high pass roll-off of the filter attenuates noise below themodulation frequency and the low pass roll-off attenuates noise abovethe modulation frequency, which also provides anti-alias filtering. Itwill be appreciated that although the band pass filter response is inthis example implemented as a single band pass filter, it could also beimplemented with separate high and low pass filters of single ormultiple stages.

The output of the band pass filter is an analogue voltage thatrepresents the carrier-modulated plethysmogram. As the detected carriermodulated plethysmogram has been band pass filtered and its high and lowfrequency content therefore attenuated, the output signal of the filteris a sine wave with a frequency equal to that of the fundamentalfrequency of the modulating carriers.

The plethysmogram is recovered by demodulating the band pass filtered,carrier modulated plethysmogram signal. Demodulation and further signalconditioning may be performed using digital signal processing. However,all of this processing can be carried out in the analogue domain usingcircuits such as a Gilbert cell I and Q mixer and a low pass filter foreach channel forming a two-channel lock-in. Therefore ananalogue-to-digital converter 607 follows the band pass filter and isused to sample the analogue voltage at the filter 606 output. It shouldbe noted that the filter is preferably the last stage before theanalogue-to-digital converter 607. This ensures the converter 607 ispresented with band pass filtered noise and not broadband white noiseand flicker noise which would be present at the output of any activecircuit stage without a limited frequency response. It will beunderstood by those skilled in the art that this will reduce the levelof noise appearing at the output of the demodulator 608.

For analogue to digital conversion and subsequent demodulation, thesample rate should preferably be at least four times a multiple integerof the modulation frequency. For example, the sample rate should be 4, 812, 16 and so forth times the modulation frequency. In a preferredarrangement, the modulation frequency is 570 Hz and the sample frequencyis 4560 Hz: sample frequency is 8 times (2×4) the modulating carrierfrequency.Sample Frequency=n*4*Modulation Frequency (where n is an integer)Minimum Sample Frequency=4*Modulation Frequency.

FIG. 11 shows a flow chart of an exemplary demodulator algorithm ascarried out in the demodulator of FIG. 2. As previously described, thedemodulator comprises a multiplexer for splitting the modulated signalinto two channels to give a first modulated input signal and a secondmodulated input signal.

Considering the first modulated signal, this is multiplied with a firstdemodulator carrier. The demodulator local oscillator (LO) is a squarewave with an amplitude of 1, a peak-to-peak amplitude of 2, andtherefore sample values of +1 and −1. Its duty cycle is 50%. Itsfrequency is equal to the modulating carrier frequency. In this examplethe modulation, and therefore demodulation frequency, is 570 Hz, and thesample rate is 4560 Hz. Therefore, the demodulation waveform compriseseight samples: four of the value +1 corresponding to the positive cycleof the carrier, and four of the value −1 corresponding with the negativecycle of the carrier. Therefore one cycle of the demodulating LO isrepresented by the samples +1, +1, +1, +1, −1, −1, −1, −1 and thispattern is repeated, ad-infinitum, to generate a continuous digitalsignal. To multiply the first modulated signal with the firstdemodulator LO and therefore obtain the I signal (step 1102), eachmeasured value of the modulated signal is multiplied with acorresponding-in-time value of the demodulator local oscillator signal:the modulated signal is multiplied by either +1 or −1. The use ofmultiplication by +1 and −1 is such that the processing is relativelysimple on chip and such an approach is easily transferred to a singleintegrated circuit if required.

Now considering the second modulated signal, this is multiplied with asecond demodulator LO. The demodulator LO is a square wave with anamplitude of 1, a peak-to-peak amplitude of 2, and therefore samplevalues of +1 and −1. Its duty cycle is 50%. Its frequency is equal tothe modulation carrier frequency. In this example the modulation, andtherefore demodulation frequency, is 570 Hz and the sample rate is 4560Hz. Therefore, the demodulation waveform consists of eight samples: fourof the value +1 corresponding to the positive cycle of the carrier, andfour of the value −1 corresponding with the negative cycle of thecarrier. However, the second demodulator carrier is phase shifted by 90degrees with respect to the first demodulator carrier. Therefore onecycle of the demodulation carrier is represented by the samples −1, −1,+1, +1, +1, +1, −1, −1 and this pattern is repeated, ad-infinitum, togenerate a continuous demodulation LO. Note that this is not the same asthe first demodulated carrier signal given above but is a 90 degreephase shifted version of it. To multiply the first modulated signal withthe first demodulator LO and therefore obtain the Q signal, eachmeasured value of the modulated signal is multiplied (step 1105) with acorresponding-in-time value of the demodulated carrier signal: it wasmultiplied by either +1 or −1.

It will now be appreciated that, in this example which uses ananalogue-to-digital converter and digital demodulator, the requirementfor sampling the band pass filtered detected signal, at a minimum offour times the modulation frequency or at a integer multiple of fourtimes the modulation frequency is so that the 90 degree phase shift canbe accurately implemented, by shifting the sampled demodulation LOone-quarter of its cycle.

Separately, the I and Q signals are each low pass filtered to removeunwanted harmonics and products of the multiplication process, anddecimated to reduced the sample rate. This is carried out by summingeach signal in eight-sample-long blocks (steps 1103 and 1106). For eachchannel, the first eight samples are summed then the second eight and soforth ad infinitum. It will be understood that this is equivalent tointegrating the I and Q signal over one cycle. It will be appreciatedthat this is an averaging process, which gives a low pass filterfrequency response and therefore attenuates the high frequencymultiplier products. It will also be appreciated that this is an averagefilter that gives a frequency response with large nulls at multiples ofthe carrier frequency. This provides good attenuation of modulationcarrier harmonics. Finally it will be appreciated that in summing eightsamples to one sample, the process also acts as a decimation stage. Thisreduced sample rate eases computational complexity of later signalprocessing stages and reduces the analogue-to-digital converter noisefloor improving signal-to-noise ratio.

The filtered and decimated I signal is now called I′. The filtered anddecimated Q signal is now called Q′.

Finally, the I′ and Q′ signals are demultiplexed back into one signal:the demodulated plethysmogram. Each I′ sample is multiplied by itself togive I′² (step 1104). Each Q′ sample is multiplied with itself to giveQ′² (step 1107). Each I′² sample is summed with its corresponding Q′²sample to give: I′²+Q′² (step 1108). Each summed sample is squarerooted: (I′²+Q′²)^(0.5) (step 1109) to give a plethysmogram sample.

The final stage in the exemplary photoplethysmograph device 600 is theblock average filter 609. The block average filter sums consecutiveblocks of 19 samples (step 1110) to give one sample. This provides thefunction of an averaging filter and decimator and its characteristicsare used to attenuate harmonically related noise, in particular thenoise generated by 60 Hz computer monitors. The averaging filter has afrequency response that gives a null (large attenuation) at multiples ofthe sampling frequency. The original sampling frequency of 4560 Hz hasbeen decimated by 8, then by 19, giving a final sample frequency of 30Hz. Therefore the averaging filter response gives large attenuation atmultiples of 30 Hz.

The light source 602 is modulated with a 570 Hz carrier. This positionsit halfway between 540 Hz (9th harmonic of 60 Hz), and 600 Hz (10thharmonic of 60 Hz). At the output of the demodulator, these harmonicsappear at 30 Hz (with all other harmonics appearing at even multiples of30 Hz). The block average filter 609 is a simple method to attenuatethis interference. The output of this final stage of filtering (steps1111, 1112) is the plethysmogram (S1(t)).

It will be appreciated that the final sample rate, and therefore thefrequency characteristics of the final stage block average filter, willdepend on the decimator ratios used in the demodulator 608 and blockaverage filter 609. Therefore, these ratios can be adjusted to giveattenuation of different harmonics by the block average filter. A rangeof values for modulation carrier frequency, sample rate and decimationratios are given in Table 1 below. A modulation carrier frequency,sample rate and decimation ratio is chosen to attenuate a given,problematic, refresh rate.

Refresh Refresh Modulation Harmonic Sample Refresh Rate Rate Carrierafter Rate Rate Harmonic Harmonic Frequency Demodulation (8 × Carrier)Decimation (Hz) (Hz) (Hz) (Hz) (Hz) (Hz) Ratio 60 540 600 570 30 4560152 70 490 560 525 35 4200 120 72 504 576 540 36 4320 120 75 525 600562.5 37.5 4500 120 85 510 595 552.5 42.5 4420 104

A typical output signal can be seen in FIG. 13. FIG. 13 a shows thecombined AC and DC components. FIG. 13 b shows the magnified ACcomponents. The higher frequency periodicity is the measured subject'spulse rate. The lower frequency periodicity is the measured subject'sbreathing rate, which was verified with a thermistor probe. Algorithmsfor determining the pulse rate are commonly found in the literature andconsist of peak detection etc.

Applications

An advantage of the plethysmograph devices described here is that areliable reflectance mode sensor can be used on many sites of the bodynot previously suitable for photoplethysmogram sensing. For example, theforehead is a highly convenient site for monitoring in harsh conditionssuch as employees working in the mining or chemical processing industrywho have to wear safety helmets. The device can be conveniently locatedin the band of the safety helmet or on the wrist under a watch or othersuch convenient places on the body. Forehead sensors and head placementhas been described in Branche et al: “Measurement Reproducibility andSensor Placement Considerations in Designing a Wearable Pulse Oximeterfor Military Applications”, IEEE 30th Annual Northeast BioengineeringConference, Springfield, Mass., United States, 2004. Their paperreported a hat mounted forehead sensor for military applications.

Another harsh environment is in the maternity suite in hospitals fornewborns that need resuscitating. Placing such a transducer on theforehead allows the medic to concentrate on the neo-natal care whilstcontinually hearing an audible bleep indicating the pulse rate. Such adevice will be highly suitable for other harsh and routine environmentsin the health and safety fields. On the other hand soft applicationsexist again for mounting in clothing for social, domestic, sports andbiometric applications.

Results

FIGS. 13, 14 and 15 show results from experiments with the exemplaryphotoplethysmograph devices previously described. Thephotoplethysmograph was used to record the plethysmogram by illuminatingthe subject's forehead. Therefore these graphs show the foreheadplethysmogram.

FIG. 13 a shows the plethysmogram AC and DC component. This is atraditional plethysmogram signal that would be expected. FIG. 13 b showsthe AC component which has been magnified. The pulsatile signal causedby the arterial pulse travelling under the sensor is clearly visible.This is superimposed on another, lower frequency, signal which has aperiod of approximately 10 seconds. This is the breathing signal causedby variations in blood volume as the subject inhales and exhales. Inthis experiment the subject breathed at a fairly constant rate anddepth, inhaling and exhaling once every 8 seconds. This is clearly seenbetween 10 and 60 seconds.

FIG. 14 a shows a magnified plethysmogram AC component. FIG. 14 b showsthe AC component after it has been band pass filtered to attenuate thepulsatile signal. The photoplethysmogram breathing signal is clearlyvisible.

FIG. 15 confirms that this low frequency AC signal is thephotoplethysmogram breathing signal. FIG. 15 a shows thephotoplethysmogram breathing signal. FIG. 15 b shows the signal from anoral thermistor. This thermistor was placed in a plastic tube throughwhich the subject breathed. As the subject exhales, air from the lungswarmed by the body causes a rise in temperature. As the subject inhales,cooler air from the room is drawn across the thermistor and the sensorrecords a drop in temperature. Thus breathing rate can be measured andcorrelated with the photoplethysmogram signal to validate thephotoplethysmogram breathing rate signal.

Inspection and comparison of FIGS. 15 a and 15 b show that theplethysmogram AC component contains both pulsatile and breathinginformation and that the exemplary photoplethysmograph detects thesesignals with ease. The two signals should be 180 degrees out of phase,which is the case. The small phase delay is caused by thermalcapacitance of the thermistor.

In a general aspect, the demodulator outputs of the photoplethysmographdevices as described herein (e.g. plethysmogram S1(t)) generally providea signal that is indicative of blood volume as a function of time. Thiscan be analysed using techniques known to the skilled person. The outputcan also be used to deduce blood constituents or blood composition. Theperiodic rise and fall in detected light intensity is assumed to besolely due to the influx of arterial blood into the tissue. By using thepeak and trough measurements, the attenuation due to the arterial bloodcan be measured. If this is performed at two different opticalwavelengths, then the oxygen saturation (the ratio of oxygenated todeoxygenated blood) can be estimated, using known techniques.

Green Light Photoplethysmography

The technique of photoplethysmography is used in pulse oximeters whichdetermine the relative oxygen saturation of blood. These devices arenormally used in transmission mode: light is used to illuminate an areaof tissue and the emergent light on the other side of the tissue isdetected and processed to determine the percentage saturation. Thistechnique is restricted to areas of skin thin enough for light to passthrough, such as the fingers, toes and ear lobes.

The choice of light wavelength in transmission mode pulse oximetry isimportant. The absorption of light by blood decreases by an order ofmagnitude from 450 nm to 600 nm, then again from 600 nm to 650 nm andbeyond. This absorption is a function of the photon path length andabsorption coefficient and is very large in transmission mode. Theresult of this is that the attenuation of light between 450 nm and 600nm is very high, to such an extent that very little light of wavelengthbelow 600 nm will pass through an appendage such as the finger, toe orearlobe. Generally light is used at 650 nm or higher.

Similarly, much of the research of photoplethysmography has been withdevices that work in the transmission mode, and hence light of awavelength greater than 600 nm has been used.

When used in reflection mode the path length, and therefore overallabsorption, is smaller. This is because the light does not pass throughan appendage, but is scattered (or reflected) from the surface layers oftissue back to the detector. This means that light of between 450 nm and600 nm can be used. However, the intensity is still very low and lownoise detection techniques are necessary to achieve an adequatesignal-to-noise ratio.

The advantage of using light of a wavelength that is strongly absorbedis because the main absorbing medium is blood. This means that a changein blood volume will cause a corresponding but larger change in theintensity of light between 450 nm and 600 nm than 600 nm and beyond.Hence, the light amplitude is modulated by the blood to a greater degreeand therefore the pulsatile component of a reflectancephotoplethysmogram signal is much larger when light between 450 nm and600 nm is used than when light above 600 nm is used. This is illustratedby FIGS. 16 and 17.

FIG. 16 shows a photoplethysmogram using green light at a wavelength of510 nm and FIG. 17 shows a photoplethysmogram using red light at awavelength of 644 nm. The scaling of the y-axis in both plots isidentical. It can be clearly seen that green light gives a largerpulsatile signal than red light. The pulse caused by the heart beat isclearly visible and of greater amplitude than when red light is used,with a corresponding improvement in signal-to-noise ratio.

Additionally, the results from the green light clearly show thebreathing signal as a low frequency base line drift. The breathingsignal was not easily observed when red light was used.

A number of the features described here can readily be used inconjunction with one another.

The use of modulated light with quadrature demodulation as described inconnection with FIG. 2, in which the demodulator is insensitive to anyphase difference between the modulation signal and the oscillator of thedemodulator offers several advantages over prior art methods. Themodulated photoplethysmogram signal can be band pass filtered at themodulating frequency to give good attenuation of DC ambient light, 100Hz fluorescent light and 60 Hz computer monitor light, and flickernoise. It therefore gives better rejection to interference than priorart schemes that use DC (unmodulated) light, or modulated light withtimeslot detection which is commonly used in pulse oximeters where it isused as a method of time division multiplexing between a red and aninfra-red LED.

Quadrature demodulation is also insensitive to the difference betweenthe phase of the modulating and demodulating carrier. This can simplifydemodulation process to a simple algorithm, with no synchronisation ofthe carriers necessary.

Quadrature demodulation can readily be used in conjunction with themultiple wavelength plethysmograph devices described here, as well aswith the pixel array devices, reflection mode devices and green lightdevices.

The combination of green light photoplethysmography and quadraturedemodulation is found to be particularly advantageous. The use of greenlight maximises the amplitude of the detected photoplethysmogram signaland modulated light with a band pass filter and quadrature demodulationminimises the effects of noise. This combination thus maximises thesignal-to-noise ratio and this means the heart rate and breathing ratecan be extracted with greater reliability. In the case of heart ratedetection it will reduce false positives or missed beats. In the case ofthe breathing signal it has clearly recovered a signal that haspreviously been difficult to detect, and the technique reduces thenumber of false positives and missed breaths.

The improved signal-to-noise ratio of the photoplethysmogram signal thusimproves the detection of the features in the signal that relate toheart beat and breathing, and therefore improves the reliability of anyalgorithm that uses these features to determine the heart and breathingrate.

In various figures, such as FIGS. 1-3, 6, 12 and 18-20, the modulatingsignals are labelled M1(t) and the demodulating signals as D1(t)indicative of a signal in the continuous time domain i.e. an analoguesignal with an amplitude that varies as a function of time. It will beunderstood that the arrangements described could readily be implementedusing a digital signal processing algorithm, e.g. in a microprocessor.In such cases, it will be understood that M1, D1 would be discretesampled signals, M1(n) and D1(n). Similarly, in FIG. 12 functionalblocks G(s) and B(s) could be represented by signal conditioningalgorithms B(z) 1201 and G(z) 1203.

Other embodiments are intentionally within the scope of the accompanyingclaims.

The invention claimed is:
 1. A photoplethysmograph device comprising: afirst light source for illuminating a target object; a modulator fordriving the first light source such that an output intensity varies as afunction of a modulation signal at a modulation frequency; a detectorfor receiving light from the target object and generating an electricaloutput as a function of intensity of the received light; and ademodulator for receiving the detector output, having at least one localoscillator and a demodulated output representative of the modulationsignal and any sidebands thereof, in which the demodulator isinsensitive to any phase difference between the modulation signal andthe local oscillator, the demodulator is configured to: multiply thedetector output with a first and second square wave output from the atleast one local oscillator to produce an I signal and Q signalrespectively; and to filter and decimate the I signal and Q signal bysumming samples in blocks corresponding with a cycle of an oscillatorfrequency to produce a filtered and decimated I′ signal and Q′ signal,and to form the demodulated output by de-multiplexing the I′ signal andQ′ signal together, wherein the demodulated output is a signalindicative of blood volume as a function of time and/or bloodcomposition, wherein the demodulator is a digital signal processingdemodulator, and the at least one local oscillator is operable toproduce the first square wave output at the oscillator frequency and ata first phase angle, and the second square wave output at the oscillatorfrequency and at a second phase angle, the second phase angle being at90 degrees to the first phase angle.
 2. The photoplethysmograph deviceof claim 1, wherein the oscillator frequency is substantially equal tothe modulation frequency.
 3. The photoplethysmograph device of claim 1further including a feedback circuit adapted to adjust output intensityof the first light source as a function of the detector output or thedemodulated output, and/or to maintain the output intensity of the lightsource at a level adequate to maintain detection of a plethysmogram fromthe demodulated output.
 4. The photoplethysmograph device of claim 1,wherein the light source has an output in a range of an optical spectrumbetween 450 nm and 600 nm.
 5. The photoplethysmograph device of claim 1further comprising: a second light source for illuminating the targetobject with light of a different wavelength than the first light source,a second modulator for driving the second light source such that outputintensity varies as a function of a second modulation signal, and asecond demodulator for producing a second demodulated outputrepresentative of the second modulation signal.
 6. Thephotoplethysmograph device of claim 5 in which the second demodulator isinsensitive to any phase difference between the modulation signal and alocal oscillator of the second demodulator.
 7. The photoplethysmographdevice of claim 5 further including means for generating, from thesecond demodulator output, a signal indicative of blood volume as afunction of time and/or blood composition.
 8. The photoplethysmographdevice of claim 1 further including a band pass filter coupled betweenthe detector output and an demodulator input having a pass band onlysufficiently wide to pass the modulation frequency and any sidebandscaused by plethysmogram amplitude modulation in the detector output. 9.The photoplethysmograph device of claim 8 in which the pass band islimited substantially to 25 Hz either side of the modulation frequencyor a block averaging frequency is selected to reject noise sources witha frequency of 60 Hz.
 10. The photoplethysmograph device of claim 1,wherein the at least one local oscillator comprises a first localoscillator producing the first square wave output, and a second localoscillator producing the second square wave output.
 11. Thephotoplethysmograph device of claim 1 wherein the light source comprisesfour light emitting diodes, arranged around the detector, with two lightemitting diodes arranged adjacent to each of two opposite edges of thedetector.
 12. The photoplethysmograph device of claim 1, furthercomprising a probe, the probe comprising the light source and detector,and the probe being located under a watch or mounted in clothing. 13.The photoplethysmograph device of claim 1, wherein the detector ispositioned at a height greater than that of the first light source so asto reduce direct coupling of light onto an active surface of thedetector.
 14. The photoplethysmograph device of claim 1, wherein thelight source comprises an active surface that emits light, and the lightsource and detector are disposed laterally adjacent to one another on asubstrate such that the respective active surfaces of the light sourceand detector can be directed towards a same point on a surface of thetarget object.
 15. A method of generating a plethysmogram comprising thesteps of: illuminating a target object with a light source; driving thelight source with a modulator such that an output intensity varies as afunction of a modulation signal at a modulation frequency; receivinglight from the target object with a detector and generating anelectrical output as a function of intensity of the received light;receiving detector output in a demodulator having a local oscillator andproducing a demodulated output representative of the modulation signaland any sidebands thereof, in which the demodulator is insensitive toany phase difference between the modulation signal and the oscillator ofthe demodulator, the demodulator is a digital signal processingdemodulator comprising: a first local oscillator producing a square waveoutput at an oscillator frequency and at a first phase angle, and asecond local oscillator producing a square wave output at the oscillatorfrequency and at a second phase angle, the second phase angle being at90 degrees to the first phase angle; and generating, from thedemodulated output, a signal indicative of blood volume as a function oftime and/or blood composition; wherein the demodulator is configured tomultiply the detector output with the output from the first and secondlocal oscillator to produce an I signal and Q signal respectively,wherein the I signal and Q signal are filtered and decimated by summingsamples in blocks corresponding with a cycle of the oscillator frequencyto produce a filtered and decimated I′ signal and Q′ signal, the I′signal and Q′ signal being demultiplexed together to form thedemodulated output.